One of the preferred methods for the commercial preparation of polyvinyl chloride consists in carrying out the polymerization of vinyl chloride in an aqueous suspension. The slurry obtained from the polymerization reactor generally has a polyvinyl chloride content in the range of 20 to 60% by weight. More usually, the polyvinyl chloride content is in the range of 30 to 50% by weight. The slurry also contains residual vinyl chloride, which is present in an amount of less than 0.1 to 5%. One means of removing the vinyl chloride is by steam distillation. While steam distillation removes a substantial amount of vinyl chloride, the polyvinyl chloride resulting from the process still has some gels or fish eyes, which are objectionable. Moreover, occasionally the steam distillation causes an increase in the gels or fish eyes.
The problem of gels or fish eyes in polyvinyl chloride is well known and understood in the art. However, in order to provide a better disclosure, a brief discussion is provided of this problem.
Polyvinyl chloride film and articles are prepared by fusing small particles of polyvinyl chloride in the presence of plasticizer. In the fusion process some particles of polyvinyl chloride absorb plasticizer at a rate different from the majority of the particles. As a result, the film or articles have imperfections such as deformations or surface roughness. The particles causing this problem are known as "gels" or "fish eyes."
My invention is directed to an improved process for removing vinyl chloride from a water-polyvinyl slurry by steam distillation wherein the polyvinyl chloride, after steam distillation, and on further processing, has no substantial increase in, or a reduced amount of, gels or fish eyes. Moreover, my process results in a product still having good heat stability and porosity properties.